Method of producing thin polarizing film

ABSTRACT

The present invention provides a method of producing a thin polarizing film having excellent optical characteristics. 
     The method of producing a thin polarizing film of the present invention includes forming a polyvinyl alcohol-based resin layer  12  on a thermoplastic resin substrate  11  to prepare a laminate  10 , subjecting the laminate  10  to underwater stretching in an aqueous solution of boric acid and stretching the laminate  10  while drying the laminate  10  after the underwater stretching.

TECHNICAL FIELD

The present invention relates to a method of producing a thin polarizing film.

BACKGROUND ART

An optical laminate having a polarizing film is placed on each of both sides of a liquid crystal cell of a liquid crystal display apparatus as a representative image display apparatus, the placement being attributable to an image-forming mode of the apparatus. In recent years, as thinning of the optical laminate having a polarizing film has been desired, there has been proposed a method involving subjecting a laminate of a thermoplastic resin substrate and a polyvinyl alcohol-based resin layer (hereinafter, referred to as “PVA-based resin layer”) to in-air stretching, and then immersing the laminate in a dyeing liquid so that a thin polarizing film may be obtained (for example, Patent Literature 1). However, such method involves a problem in that optical characteristics (such as polarization degree) of the thin polarizing film to be obtained are insufficient.

CITATION LIST Patent Literature

-   [PTL 1]JP 2001-343521 A

SUMMARY OF INVENTION Technical Problem

The present invention has been made to solve the conventional problem, and a main object of the present invention is to provide a method of producing a thin polarizing film having excellent optical characteristics.

Means for solving the Problems

According to one aspect of the present invention, a method of producing a thin polarizing film is provided. The method of producing a thin polarizing film includes forming a polyvinyl alcohol-based resin layer on a thermoplastic resin substrate to prepare a laminate, subjecting the laminate to underwater stretching in an aqueous solution of boric acid and stretching the laminate while drying the laminate after the underwater stretching.

In one embodiment of the invention, a stretching ratio in the drying is 1.01 times to 1.5 times.

In another embodiment of the invention, a maximum stretching ratio of the laminate is 5.0 times or more.

Instill another embodiment of the invention, the thermoplastic resin substrate includes an amorphous polyethylene terephthalate-based resin.

In still another embodiment of the invention, the method of producing a thin polarizing film further includes subjecting the laminate to in-air stretching at 95° C. or more before the underwater stretching.

According to another aspect of the invention, a thin polarizing film is provided. The thin polarizing film is obtained by the method of producing a thin polarizing film.

According to still another aspect of the invention, an optical laminate is provided. The optical laminate includes the thin polarizing film.

Advantageous Effects of Invention

According to the present invention, the underwater stretching (boric acid underwater stretching) is performed through the use of the aqueous solution of boric acid, to thereby allow the laminate having formed therein the PVA-based resin layer to be stretched at a high ratio and in a favorable manner. Further, the laminate is stretched while being dried, to thereby allow the laminate to undergo additional shrinkage (neck-in) in a direction (such as widthwise direction) substantially perpendicular to the direction of the stretching, which additionally raises the orientation property of the PVA-based resin layer. As a result, a thin polarizing film extremely excellent in optical characteristics (such as polarization degree) can be produced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of a laminate according to a preferred embodiment of the present invention.

FIG. 2 is a schematic view illustrating an example of a method of producing a thin polarizing film of the present invention.

FIG. 3 are each a schematic sectional view of an optical film laminate according to a preferred embodiment of the present invention.

FIG. 4 are each a schematic sectional view of an optical functional film laminate according to another preferred embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention are described. However, the present invention is not limited to these embodiments.

A. Production Method

A method of producing a thin polarizing film of the present invention includes: forming a polyvinyl alcohol-based resin layer on a thermoplastic resin substrate to prepare a laminate (step A); subjecting the laminate to underwater stretching in an aqueous solution of boric acid (step B); and stretching the laminate while drying the laminate after the step B (step C). Hereinafter, the respective steps are described.

A-1. Step A

FIG. 1 is a schematic sectional view of a laminate according to a preferred embodiment of the present invention. A laminate 10 has a thermoplastic resin substrate 11 and a PVA-based resin layer 12, and is prepared by forming the PVA-based resin layer 12 on the thermoplastic resin substrate 11. Any appropriate method can be adopted as a method of forming the PVA-based resin layer 12. The PVA-based resin layer 12 is preferably formed by applying an application liquid containing a PVA-based resin onto the thermoplastic resin substrate 11 and drying the liquid.

Any appropriate material can be adopted as a constituent material for the thermoplastic resin substrate. Amorphous (uncrystallized) polyethylene terephthalate-based resins are each preferably used as the constituent material for the thermoplastic resin substrate. Of those, a noncrystalline (hard-to-crystallize) polyethylene terephthalate-based resin is particularly preferably used. Specific examples of the noncrystalline polyethylene terephthalate-based resin include a copolymer further containing isophthalic acid as a dicarboxylic acid and a copolymer further containing cyclohexane dimethanol as a glycol.

The thermoplastic resin substrate absorbs water in the step B to be described later and the water serves a plastic function so that the substrate can plasticize. As a result, a stretching stress can be significantly reduced. Accordingly, the stretching can be performed at a high ratio and the stretchability of the thermoplastic resin substrate can be more excellent than that at the time of in-air stretching. As a result, a thin polarizing film having excellent optical characteristics (such as polarization degree) can be produced. In one embodiment, the percentage of water absorption of the thermoplastic resin substrate is preferably 0.2% or more, more preferably 0.3% or more. Meanwhile, the percentage of water absorption of the thermoplastic resin substrate is preferably 3.0% or less, more preferably 1.0% or less. The use of such thermoplastic resin substrate can prevent, for example, the following inconvenience. The dimensional stability of the thermoplastic resin substrate remarkably reduces at the time of the production and hence the external appearance of the thin polarizing film to be obtained deteriorates. In addition, the use can prevent the rupture of the substrate at the time of the underwater stretching and the release of the PVA-based resin layer from the thermoplastic resin substrate. It should be noted that the percentage of water absorption is a value determined in conformity with JIS K 7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 170° C. or less. The use of such thermoplastic resin substrate can sufficiently secure the stretch ability of the laminate while suppressing the crystallization of the PVA-based resin layer. Further, the glass transition temperature is more preferably 120° C. or less in consideration of the plasticization of the thermoplastic resin substrate by water and favorable performance of the underwater stretching. In one embodiment, the glass transition temperature of the thermoplastic resin substrate is preferably 60° C. or more. The use of such thermoplastic resin substrate prevents an inconvenience such as the deformation of the thermoplastic resin substrate (e.g., the occurrence of unevenness, a slack, or a wrinkle) during the application and drying of the application liquid containing the PVA-based resin, thereby enabling favorable production of the laminate. In addition, the use enables favorable stretching of the PVA-based resin layer at a suitable temperature (e.g., about 60° C.). In another embodiment, a glass transition temperature lower than 60° C. is permitted as long as the thermoplastic resin substrate does not deform during the application and drying of the application liquid containing the PVA-based resin.

The glass transition temperature of the thermoplastic resin substrate can be adjusted by, for example, introducing a denaturation group into the constituent material or heating the substrate constituted of a crystallization material. It should be noted that the glass transition temperature (Tg) is a value determined in conformity with JIS K 7121.

The thickness of the thermoplastic resin substrate before the stretching is preferably 20 μm to 300 μm, more preferably 50 μm to 200 μm. When the thickness is less than 20 μm, it may be difficult to form the PVA-based resin layer. When the thickness exceeds 300 μm, in, for example, the step B, it may take a long time for the thermoplastic resin substrate to absorb water, and an excessively large load may be needed in the stretching.

Any appropriate resin can be adopted as the PVA-based resin. Examples of the resin include a polyvinyl alcohol and an ethylene-vinyl alcohol copolymer. The polyvinyl alcohol is obtained by saponifying a polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying an ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is typically 85 mol % to 100 mol %, preferably 95.0 mol % to 99.95 mol %, more preferably 99.0 mol % to 99.93 mol %. The saponification degree can be determined in conformity with JIS K 6726-1994. The use of the PVA-based resin having such saponification degree can provide a thin polarizing film excellent in durability. When the saponification degree is excessively high, the resin may gel.

The average polymerization degree of the PVA-based resin can be appropriately selected depending on purposes. The average polymerization degree is typically 1,000 to 10,000, preferably 1,200 to 4,500, more preferably 1,500 to 4,300. It should be noted that the average polymerization degree can be determined in conformity with JIS K 6726-1994.

The application liquid is representatively a solution prepared by dissolving the PVA-based resin in a solvent. Examples of the solvent include water, dimethylsulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. One kind of those solvents can be used alone, or two or more kinds thereof can be used in combination. Of those, water is preferred. The concentration of the PVA-based resin of the solution is preferably 3 parts by weight to 20 parts by weight with respect to 100 parts by weight of the solvent. At such resin concentration, a uniform coating film in close contact with the thermoplastic resin substrate can be formed.

The application liquid may be compounded with an additive. Examples of the additive include a plasticizer and a surfactant. Examples of the plasticizer include polyhydric alcohols such as ethylene glycol and glycerin. Examples of the surfactant include nonionic surfactants. Such additive can be used for the purpose of additionally improving the uniformity, dyeing property, or stretchability of the PVA-based resin layer to be obtained.

Any appropriate method can be adopted as a method of applying the application liquid. Examples of the method include a roll coating method, a spin coating method, a wire bar coating method, a dip coating method, a die coating method, a curtain coating method, a spray coating method, and a knife coating method (comma coating method or the like).

The application liquid is preferably applied and dried at a temperature of 50° C. or more.

The thickness of the PVA-based resin layer before the stretching is preferably 3 μm to 20 μm.

The thermoplastic resin substrate may be subjected to a surface treatment (such as a corona treatment) before the formation of the PVA-based resin layer. Alternatively, an easy-adhesion layer may be formed on the thermoplastic resin substrate. Performing such treatment can improve adhesiveness between the thermoplastic resin substrate and the PVA-based resin layer.

A-2. Step B

In the step B, the laminate is subjected to underwater stretching (boric acid underwater stretching). According to the underwater stretching, the stretching can be performed at a temperature lower than the glass transition temperature (representatively about 80° C.) of each of the thermoplastic resin substrate and the PVA-based resin layer, and hence the PVA-based resin layer can be stretched at a high ratio while its crystallization is suppressed. As a result, a thin polarizing film having excellent optical characteristics (such as polarization degree) can be produced.

Any appropriate method may be adopted as a method of stretching the laminate. Specifically, fixed-end stretching may be adopted, or free-end stretching (such as a method involving passing the laminate between rolls having different peripheral speeds to uniaxially stretch the laminate) may be adopted. The stretching of the laminate may be performed in one stage, or may be performed in a plurality of stages. When the stretching is performed in a plurality of stages, a stretching ratio to be described later is the product of stretching ratios in the respective stages.

The underwater stretching is preferably performed by immersing the laminate in an aqueous solution of boric acid (boric acid underwater stretching). The use of the aqueous solution of boric acid as a stretching bath can impart, to the PVA-based resin layer, rigidity enough to withstand a tension to be applied at the time of the stretching and such water resistance that the layer does not dissolve in water. Specifically, boric acid can produce a tetrahydroxyborate anion in the aqueous solution to cross-link with the PVA-based resin through a hydrogen bond. As a result, the PVA-based resin layer can be favorably stretched with the aid of the rigidity and the water resistance imparted thereto, and hence a thin polarizing film having excellent optical characteristics (such as polarization degree) can be produced.

The aqueous solution of boric acid is preferably obtained by dissolving boric acid and/or a borate in water as a solvent. The concentration of boric acid is preferably 1 part by weight to 10 parts by weight with respect to 100 parts by weight of water. Setting the concentration of boric acid to 1 part by weight or more can effectively suppress the dissolution of the PVA-based resin layer, thereby enabling the production of a thin polarizing film having additionally high characteristics. It should be noted that an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaric aldehyde, or the like as well as boric acid or the borate in the solvent may also be used.

When the PVA-based resin layer has been caused to adsorb a dichromatic substance (representatively iodine) in advance through the dyeing step to be described later, the stretching bath (aqueous solution of boric acid) is preferably compounded with an iodide. Compounding the bath with the iodide can suppress the elution of iodine which the PVA-based resin layer has been caused to adsorb. Examples of the iodide include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Of those, potassium iodide is preferred. The concentration of the iodide is preferably 0.05 part by weight to 15 parts by weight, more preferably 0.5 part by weight to 8 parts by weight with respect to 100 parts by weight of water.

A stretching temperature (liquid temperature of stretching bath) in the step B is preferably 40° C. to 85° C., more preferably 50° C. to 85° C. At such temperature, the PVA-based resin layer can be stretched at a high ratio while its dissolution is suppressed. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60° C. or more in relation to the formation of the PVA-based resin layer. In this case, when the stretching temperature falls short of 40° C., there is a risk that the stretching cannot be favorably performed even in consideration of the plasticization of the thermoplastic resin substrate by water. On the other hand, as the temperature of the stretching bath increases, the solubility of the PVA-based resin layer is raised and hence excellent optical characteristics may not be obtained. The laminate is preferably immersed in the stretching bath for a time of 15 seconds to 5 minutes.

A stretching ratio in this step (stretching ratio in underwater stretching) is, for example, set in consideration of a maximum stretching ratio and a stretching ratio in the step C to be described later. When a stretching step different from the step B and the step C is performed, the stretching ratio may be set in consideration of a stretching ratio in the different step as well. It should be noted that the stretching ratio in this step is preferably 1.25 times or more.

A-3. Step C

After the step B, in the step C, the laminate is stretched while being dried. When being stretched while being dried, the laminate can undergo additional shrinkage (neck-in) in a direction (such as widthwise direction) substantially perpendicular to the direction of the stretching to additionally enhance the orientation property of the PVA-based resin layer. As a result, the orientation property of an iodine complex in the PVA-based resin layer can be improved, and hence a thin polarizing film extremely excellent in optical characteristics (such as polarization degree) can be produced.

As in the step B, a method for the stretching in this step may be fixed-end stretching, or may be free-end stretching (such as a method involving passing the laminate between rolls having different peripheral speeds to uniaxially stretch the laminate). In addition, the stretching may be performed in one stage, or may be performed in a plurality of stages. When the stretching is performed in a plurality of stages, a stretching ratio to be described later is the product of stretching ratios in the respective stages. It is preferred that a stretching direction in this step be substantially the same as the stretching direction in the step B.

The free-end stretching generally means a stretching method involving stretching in only one direction. In this regard, when a film is stretched in one given direction, the film may shrink in a direction substantially perpendicular to the direction of the stretching. A method involving stretching without suppressing the shrinkage is referred to as free-end stretching. On the other hand, a method involving stretching while suppressing the shrinkage of the film in the direction substantially perpendicular to the direction of the stretching is generally referred to as fixed-end stretching. Examples of the fixed-end stretching method include a method involving stretching in a conveying direction in roll stretching using conveying rolls with a distance between the rolls being set short, and a method involving stretching in a conveying direction under a state in which end portions of a film in a direction (widthwise direction) substantially perpendicular to the conveying direction are fixed with, for example, chucks of a stretching machine. Those methods each allow a shrinkage ratio in the widthwise direction to be controlled. Specifically, in the roll stretching, the amount of shrinkage can be reduced to a greater extent as the distance between the rolls is set shorter, while the amount of shrinkage is increased to a greater extent and brought closer to the amount of shrinkage in the free-end stretching as the distance between the rolls is set longer. When stretching is performed while the end portions of the film are fixed with chucks, the shrinkage ratio in the widthwise direction can be controlled by appropriately decreasing a distance between the chucks simultaneously with the stretching.

A drying temperature (stretching temperature) is preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate. For example, the drying temperature is preferably 60° C. to 120° C., more preferably 70° C. to 100° C. When being stretched while being dried at such temperature, the laminate can be favorably stretched and shrunk. In addition, an inconvenience such as deterioration of optical characteristics during the drying can be prevented.

In this step, the stretching is preferably performed until a maximum stretching ratio is achieved. The term “maximum stretching ratio” as used in the description refers to a stretching ratio immediately before the rupture of the laminate. The stretching ratio at which the laminate ruptures is separately identified and a value lower than the value by 0.2 is the maximum stretching ratio. Specifically, the stretching ratio in this step is preferably 1.01 times to 1.5 times, more preferably 1.01 times to 1.25 times. The maximum stretching ratio is preferably 5.0 times or more with respect to the original length of the laminate. Performing the step B enables stretching at a high ratio as described above, and hence a thin polarizing film having excellent optical characteristics (such as polarization degree) can be produced.

A-4. Any Other Step

The method of producing a thin polarizing film according to the present invention may include any other step except the step A, the step B, and the step C. Examples of the other step include an insolubilizing step, a dyeing step, a cross-linking step, the stretching step different from the step B and the step C, a washing step, and a drying (moisture content adjusting) step. The other step may be performed at any appropriate timing.

The insolubilizing step is representatively performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. Water resistance can be imparted to the PVA-based resin layer by subjecting the layer to an insolubilizing treatment. The concentration of the aqueous solution of boric acid is preferably 1 part by weight to 4 parts by weight with respect to 100 parts by weight of water. The liquid temperature of an insolubilizing bath (the aqueous solution of boric acid) is preferably 20° C. to 40° C. The insolubilizing step is preferably performed after the preparation of the laminate and before the dyeing step or the step B.

The dyeing step is representatively a step of dyeing the PVA-based resin layer with a dichromatic substance. The dyeing is preferably performed by causing the PVA-based resin layer to adsorb the dichromatic substance. A method for the adsorption is, for example, a method involving immersing the PVA-based resin layer (laminate) in a dyeing liquid containing the dichromatic substance, a method involving applying the dyeing liquid onto the PVA-based resin layer, or a method involving spraying the dyeing liquid onto the PVA-based resin layer. Of those, a method involving immersing the laminate in the dyeing liquid containing the dichromatic substance is preferred. This is because the dichromatic substance can favorably adsorb to the layer.

Examples of the dichromatic substance include iodine and a dichromatic dye. Of those, iodine is preferred. When iodine is used as the dichromatic substance, the dyeing liquid is an aqueous solution of iodine. The compounding amount of iodine is preferably 0.1 part by weight to 0.5 part by weight with respect to 100 parts by weight of water. The aqueous solution of iodine is preferably compounded with an iodide in order that the solubility of iodine in water may be increased. Specific examples of the iodide are as described above. The compounding amount of the iodide is preferably 0.02 part by weight to 20 parts by weight, more preferably 0.1 part by weight to 10 parts by weight with respect to 100 parts by weight of water. The liquid temperature of the dyeing liquid at the time of the dyeing is preferably 20° C. to 50° C. in order that the dissolution of the PVA-based resin may be suppressed. When the PVA-based resin layer is immersed in the dyeing liquid, an immersion time is preferably 5 seconds to 5 minutes in order that the transmittance of the PVA-based resin layer may be secured. In addition, the dyeing conditions (the concentration, the liquid temperature, and the immersion time) can be set so that the polarization degree or single axis transmittance of the polarizing film to be finally obtained may fall within a predetermined range. In one embodiment, the immersion time is set so that the polarization degree of the polarizing film to be obtained may be 99.98% or more. In another embodiment, the immersion time is set so that the single axis transmittance of the polarizing film to be obtained may be 40% to 44%.

The dyeing step is preferably performed before the step B.

The cross-linking step is representatively performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. Water resistance can be imparted to the PVA-based resin layer by subjecting the layer to a cross-linking treatment. The concentration of the aqueous solution of boric acid is preferably 1 part by weight to 4 parts by weight with respect to 100 parts by weight of water. In addition, when the cross-linking step is performed after the dyeing step, the solution is preferably further compounded with an iodide. Compounding the solution with the iodide can suppress the elution of iodine which the PVA-based resin layer has been caused to adsorb. The compounding amount of the iodide is preferably 1 part by weight to 5 parts by weight with respect to 100 parts by weight of water. Specific examples of the iodide are as described above. The liquid temperature of a cross-linking bath (the aqueous solution of boric acid) is preferably 20° C. to 50° C.

The cross-linking step is preferably performed before the step B. In a preferred embodiment, the dyeing step, the cross-linking step, and the step B are performed in the stated order.

The stretching step different from the step B or the step C is, for example, a step of subjecting the laminate to in-air stretching at a high temperature (e.g., 95° C. or more). Such in-air stretching step is preferably performed before the step B (boric acid underwater stretching) and the dyeing step. Such in-air stretching step is hereinafter referred to as “preliminary in-air stretching” because the step can be ranked as stretching preliminary or auxiliary to the boric acid underwater stretching.

When the preliminary in-air stretching is combined, the laminate can be stretched at an additionally high ratio in some cases. As a result, a thin polarizing film having additionally excellent optical characteristics (such as a polarization degree) can be produced. For example, when a polyethylene terephthalate-based resin is used as the thermoplastic resin substrate, the thermoplastic resin substrate can be stretched favorably, while its orientation is suppressed, by a combination of the preliminary in-air stretching. As the orientation property of the thermoplastic resin substrate is raised, its stretching tension increases and hence it becomes difficult to stably stretch the substrate or the thermoplastic resin substrate ruptures. Accordingly, the laminate can be stretched at an additionally high ratio by stretching the thermoplastic resin substrate while suppressing its orientation.

In addition, when the preliminary in-air stretching is combined, the orientation property of the PVA-based resin is improved and hence the orientation property of the PVA-based resin can be improved even after the boric acid underwater stretching. Specifically, the orientation property of the PVA-based resin is improved in advance by the preliminary in-air stretching so that the PVA-based resin may easily cross-link with boric acid during the boric acid underwater stretching. Then, the stretching is performed in a state where boric acid serves as a junction, and hence the orientation property of the PVA-based resin is assumed to be high even after the boric acid underwater stretching. As a result, a thin polarizing film having excellent optical characteristics (such as a polarization degree) can be produced.

As with the step B, a stretching method for the preliminary in-air stretching may be fixed-end stretching, or may be free-end stretching (such as a method involving passing the laminate between rolls having different peripheral speeds to uniaxially stretch the laminate). In addition, the stretching may be performed in one stage, or may be performed in a plurality of stages. When the stretching is performed in a plurality of stages, a stretching ratio to be described later is the product of stretching ratios in the respective stages. It is preferred that a stretching direction in the step be substantially the same as the stretching direction in the step B.

The stretching ratio in the preliminary in-air stretching is preferably 3.5 times or less. A stretching temperature in the preliminary in-air stretching is preferably equal to or higher than the glass transition temperature of the PVA-based resin. The stretching temperature is preferably 95° C. to 150° C. It should be noted that the maximum stretching ratio when the preliminary in-air stretching is combined is preferably 5.0 times or more, more preferably 5.5 times or more, still more preferably 6.0 times or more with respect to the original length of the laminate.

The washing step is representatively performed by immersing the PVA-based resin layer in an aqueous solution of potassium iodide. The washing step is preferably performed between the step B and the step C.

FIG. 2 is a schematic view illustrating an example of a method of producing a thin polarizing film according to the present invention. A laminate 10 is fed from a feeding portion 100, and is then immersed in a bath 110 of an aqueous solution of boric acid by rolls 111 and 112 (insolubilizing step). After that, the laminate is immersed in a bath 120 of an aqueous solution of a dichromatic substance (iodine) and potassium iodide by rolls 121 and 122 (dyeing step). Next, the laminate is immersed in a bath 130 of an aqueous solution of boric acid and potassium iodide by rolls 131 and 132 (cross-linking step). After that, the laminate 10 is stretched through the application of a tension in its longitudinal direction (lengthwise direction) with rolls 141 and 142 having different speed ratios while being immersed in a bath 140 of an aqueous solution of boric acid (step B). The laminate 10 subjected to the stretching treatment is immersed in a bath 150 of an aqueous solution of potassium iodide by rolls 151 and 152 (washing step), and is then subjected to a drying step 160. In the drying step 160, the laminate is stretched through the application of a tension in its longitudinal direction (lengthwise direction) with rolls (not shown) having different speed ratios while being dried with warm air (step C). After that, the laminate is wound by a winding portion 170.

B. Thin Polarizing Film

A thin polarizing film of the present invention is obtained by the production method. The thin polarizing film of the present invention is substantially a PVA-based resin film that adsorbs and orients a dichromatic substance. The thickness of the thin polarizing film is preferably 10 μm or less, more preferably 7 μm or less, still more preferably 5 μm or less. Meanwhile, the thickness of the thin polarizing film is preferably 0.5 μm or more, more preferably 1.5 μm or more. The thin polarizing film preferably shows absorption dichroism at any wavelength in the wavelength range of 380 nm to 780 nm. The single axis transmittance of the thin polarizing film is preferably 40.0% or more, more preferably 41.0% or more, still more preferably 42.0% or more. The polarization degree of the thin polarizing film is preferably 99.8% or more, more preferably 99.9% or more, still more preferably 99.95% or more.

Any appropriate method can be adopted as a usage of the thin polarizing film. Specifically, the thin polarizing film may be used in a state of being integrated with the thermoplastic resin substrate, or may be used after having been transferred from the thermoplastic resin substrate onto any other member.

C. Optical Laminate

An optical laminate of the present invention has the thin polarizing film. FIGS. 3A and 3B are each a schematic sectional view of an optical film laminate according to a preferred embodiment of the present invention. An optical film laminate 100 has a thermoplastic resin substrate 11′, a thin polarizing film 12′, a pressure-sensitive adhesive layer 13, and a separator 14 in the stated order. An optical film laminate 200 has the thermoplastic resin substrate 11′, the thin polarizing film 12′, an adhesive layer 15, an optical functional film 16, the pressure-sensitive adhesive layer 13, and the separator 14 in the stated order. In this embodiment, the thermoplastic resin substrate is directly used as an optical member without being released from the resultant thin polarizing film 12′. The thermoplastic resin substrate 11′ can function as, for example, a protective film for the thin polarizing film 12′.

FIGS. 4A and 4B are each a schematic sectional view of an optical functional film laminate according to another preferred embodiment of the present invention. An optical functional film laminate 300 has the separator 14, the pressure-sensitive adhesive layer 13, the thin polarizing film 12′, the adhesive layer 15, and the optical functional film 16 in the stated order. An optical functional film laminate 400 has, in addition to the construction of the optical functional film laminate 300, a second optical functional film 16′ provided between the thin polarizing film 12′ and the separator 14 through the pressure-sensitive adhesive layer 13. In this embodiment, the thermoplastic resin substrate has been removed.

The lamination of the respective layers constructing the optical laminate of the present invention is not limited to the illustrated examples, and any appropriate pressure-sensitive adhesive layer or adhesive layer is used. The pressure-sensitive adhesive layer is representatively formed of an acrylic pressure-sensitive adhesive. The adhesive layer is representatively formed of a vinyl alcohol-based adhesive. The optical functional film can function as, for example, a protective film for a polarizing film or a retardation film.

EXAMPLES

Hereinafter, the present invention is specifically described by way of examples. However, the present invention is not limited by these examples. It should be noted that methods of measuring the respective characteristics are as described below.

1. Thickness

Measurement was performed with a digital micrometer (manufactured by Anritsu Corporation, product name: “KC-351C”).

2. Percentage of Water Absorption of Thermoplastic Resin Substrate

Measurement was performed in conformity with JIS K 7209.

3. Glass Transition Temperature (Tg) of Thermoplastic Resin Substrate

Measurement was performed in conformity with JIS K 7121.

Example 1 Step A

An amorphous polyethylene terephthalate (A-PET) film (manufactured by Mitsubishi Plastics, Inc., trade name “NOVACLEAR,” thickness: 100 μm) having a percentage of water absorption of 0.60% and a glass transition temperature (Tg) of 80° C. was used as a thermoplastic resin substrate.

An aqueous solution of a polyvinyl alcohol (PVA) resin (manufactured by The Nippon Synthetic Chemical Industry Co., Ltd., trade name “Gohsenol (trademark) NH-26”) having a polymerization degree of 2,600 and a saponification degree of 99.9% was applied to one surface of the thermoplastic resin substrate at 60° C., and was then dried at 60° C. so that a PVA-based resin layer having a thickness of 7 μm was formed. Thus, a laminate was prepared.

The resultant laminate was immersed in an insolubilizing bath having a liquid temperature of 30° C. (an aqueous solution of boric acid obtained by compounding 100 parts by weight of water with 3 parts by weight of boric acid) for 30 seconds (insolubilizing step).

Next, the laminate was immersed in a dyeing bath having a liquid temperature of 30° C. (an aqueous solution of iodine obtained by compounding 100 parts by weight of water with 0.2 part by weight of iodine and 1.4 parts by weight of potassium iodide) so that the single axis transmittance (Ts) of a thin polarizing film to be finally obtained was 40 to 44% (dyeing step).

Next, the laminate was immersed in a cross-linking bath having a liquid temperature of 30° C. (an aqueous solution of boric acid obtained by compounding 100 parts by weight of water with 3 parts by weight of potassium iodide and 3 parts by weight of boric acid) for 60 seconds (cross-linking step).

After that, the laminate was subjected to free-end uniaxial stretching in its longitudinal direction (lengthwise direction) between rolls having different peripheral speeds while being immersed in an aqueous solution of boric acid having a liquid temperature of 75° C. (an aqueous solution obtained by compounding 100 parts by weight of water with 4 parts by weight of boric acid and 5 parts by weight of potassium iodide) (step B). The stretching ratio in this case was set to 4 times.

After that, the laminate was immersed in a washing bath (an aqueous solution obtained by compounding 100 parts by weight of water with 4 parts by weight of potassium iodide) for 5 seconds (washing step).

After that, the laminate was subjected to free-end uniaxial stretching in its longitudinal direction immediately before its rupture while being dried with warm air at 80° C. (step C). The stretching ratio in this case was 1.25 times.

Thus, an optical film laminate having a thin polarizing film having a thickness of 3 μm formed on a thermoplastic resin substrate was obtained.

Example 2 Step A

An isophthalic acid-copolymerized polyethylene terephthalate film (thickness: 200 μm) copolymerized with 6 mol % of isophthalic acid and having a percentage of water absorption of 0.75% and a Tg of 75° C. was used as a thermoplastic resin substrate.

An aqueous solution having a concentration of 4 to 5 wt % obtained by dissolving PVA resin powder having a polymerization degree of 4,200 and a saponification degree of 99.2% in water was applied onto one surface of a thermoplastic resin substrate, and was then dried at a temperature of 50 to 60° C. so that a PVA-based resin layer having a thickness of 9 μm was formed. Thus, a laminate was prepared.

The resultant laminate was subjected to free-end uniaxial stretching in its longitudinal direction (lengthwise direction) at 1.8 times in an oven at 130° C. between rolls having different peripheral speeds (preliminary in-air stretching step).

After that, the laminate was immersed in an insolubilizing bath having a liquid temperature of 30° C. (an aqueous solution of boric acid obtained by compounding 100 parts by weight of water with 3 parts by weight of boric acid) for 30 seconds (insolubilizing step).

Next, the laminate was immersed in a dyeing liquid having a liquid temperature of 30° C., having an iodine concentration of 0.12 to 0.25 wt %, and containing potassium iodide so that the single axis transmittance of a polarizing film to be finally obtained was 40 to 44% (dyeing step). The compounding ratio of iodine to potassium iodide in this case was set to 1:7.

Next, the laminate was immersed in a cross-linking bath having a liquid temperature of 40° C. (an aqueous solution of boric acid obtained by compounding 100 parts by weight of water with 3 parts by weight of potassium iodide and 3 parts by weight of boric acid) for 60 seconds (cross-linking step).

After that, the laminate was subjected to free-end uniaxial stretching in its longitudinal direction (lengthwise direction) between rolls having different peripheral speeds while being immersed in an aqueous solution of boric acid having a liquid temperature of 75° C. (an aqueous solution obtained by compounding 100 parts by weight of water with 4 parts by weight of boric acid and 5 parts by weight of potassium iodide) (step B). The stretching ratio in this case was set to 2.78 times.

After that, the laminate was immersed in a washing bath (an aqueous solution obtained by compounding 100 parts by weight of water with 4 parts by weight of potassium iodide) for 5 seconds (washing step).

After that, the laminate was subjected to free-end uniaxial stretching in its longitudinal direction immediately before its rupture while being dried with warm air at 80° C. (step C). The stretching ratio in this case was 1.2 times.

Thus, an optical film laminate having a thin polarizing film having a thickness of 3.5 μm formed on a thermoplastic resin substrate was obtained.

Example 3

An optical film laminate was obtained in the same manner as in Example 2 except that the stretching ratio in the step B was changed to 3.3 times. The stretching ratio in the step C was 1.01 times and the thickness of the thin polarizing film formed on the thermoplastic resin substrate was 3.5 μm.

Example 4

An optical film laminate was obtained in the same manner as in Example 2 except that the stretching ratio in the step B was changed to 2.69 times and the drying temperature in the step C was changed to 60° C. The stretching ratio in the step C was 1.2 times and the thickness of the thin polarizing film formed on the thermoplastic resin substrate was 3.5 μm.

Example 5

An optical film laminate was obtained in the same manner as in Example 2 except that the stretching ratio in the step B was changed to 2.41 times and the drying temperature in the step C was changed to 120° C. The stretching ratio in the step C was 1.2 times and the thickness of the thin polarizing film formed on the thermoplastic resin substrate was 3.5 μm.

Example 6

An optical film laminate was obtained in the same manner as in Example 2 except that the stretching ratio in the step B was changed to 2.06 times. The stretching ratio in the step C was 1.4 times and the thickness of the thin polarizing film formed on the thermoplastic resin substrate was 3.5 μm.

Comparative Example 1

A laminate prepared in the same manner as in Example 1 was subjected to in-air stretching in an oven at 110° C. immediately before the rupture of the laminate. The stretching ratio (maximum stretching ratio) in this case was 4.0 times.

After that, the dyeing step, the cross-linking step, and the washing step were performed in the stated order in the same manner as in Example 1, and the resultant was then dried with warm air at 60° C. Thus, a thin polarizing film was obtained. The thickness of the resultant thin polarizing film was 4 μm.

Reference Example 1

A thin polarizing film was obtained in the same manner as in Example 1 except that the laminate was subjected to underwater stretching in an aqueous solution of boric acid immediately before its rupture and dried with warm air at 60° C. The stretching ratio (maximum stretching ratio) in the underwater stretching was 5.0 times, and the thickness of the resultant thin polarizing film was 3 μm.

The polarization degree of each of the resultant polarizing films was measured. A method of measuring the polarization degree is as described below. Table 1 shows the results of the measurement together with the width retention ratio.

(Method of Measuring Polarization Degree)

The single axis transmittance (Ts), parallel transmittance (Tp), and crossed transmittance (Tc) of each polarizing film were measured with an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, product name “V7100”), and then its polarization degree (P) was determined from the following equation.

Polarization degree(P)(%)={(Tp−Tc)/(Tp+Tc)}^(1/2)×100

It should be noted that the Ts, the Tp, and the Tc are Y values measured with the two-degree field of view (C light source) of JIS Z 8701 and subjected to visibility correction.

TABLE 1 Maximum Width stretching retention Stretching conditions ratio Ts P ratio Mode and stretching temperature (times) (%) (%) (%) Example 1 Underwater at 75° C.→in air at 80° C. 5 42.5 99.970 38.5 (1.25 times) Example 2 In air at 130° C.→underwater at 6 42.5 99.997 34.5 75° C.→in air at 80° C. (1.2 times) Example 3 In air at 130° C.→underwater at 6 42.5 99.995 38.3 75° C.→in air at 80° C. (1.01 times) Example 4 In air at 130° C.→underwater at 5.8 42.5 99.990 40.5 75° C.→in air at 60° C. (1.2 times) Example 5 In air at 130° C.→underwater at 5.2 42.5 99.991 39.0 75° C.→in air at 120° C. (1.2 times) Example 6 In air at 130° C.→underwater at 5.2 42.5 99.990 41.2 75° C.→in air at 80° C. (1.4 times) Comparative In air at 110° C. 4 42.5 98.973 42.0 Example 1 Reference Underwater at 75° C. 5 42.5 99.948 41.0 Example 1

In each of Examples, a thin polarizing film significantly excellent in polarization degree as compared to Comparative Example 1 was able to be produced. This is probably because the stretching was able to be performed at a high ratio and the shrinkage ratio in the widthwise direction was high.

INDUSTRIAL APPLICABILITY

The thin polarizing film of the present invention has higher polarization performance than a conventional thin polarizing film. Thus, according to the present invention, the thin polarizing film can be applied to a liquid crystal panel of a liquid crystal television, a liquid crystal display, a mobile phone, a digital camera, a video camera, a handheld game console, a car navigation system, a copy machine, a printer, a fax machine, a watch, a microwave oven, or the like.

REFERENCE SIGNS LIST

-   -   10 laminate     -   11 thermoplastic resin substrate     -   12 PVA-based resin layer 

1. A method of producing a thin polarizing film, comprising: forming a polyvinyl alcohol-based resin layer on a thermoplastic resin substrate to prepare a laminate; subjecting the laminate to underwater stretching in an aqueous solution of boric acid; and stretching the laminate while drying the laminate after the underwater stretching.
 2. A method of producing a thin polarizing film according to claim 1, wherein a stretching ratio in the drying is 1.01 times to 1.5 times.
 3. A method of producing a thin polarizing film according to claim 1, wherein a maximum stretching ratio of the laminate is 5.0 times or more.
 4. A method of producing a thin polarizing film according to claim 1, wherein the thermoplastic resin substrate comprises an amorphous polyethylene terephthalate-based resin.
 5. A method of producing a thin polarizing film according to claim 1, further comprising subjecting the laminate to in-air stretching at 95° C. or more before the underwater stretching.
 6. A thin polarizing film, which is obtained by the method of producing a thin polarizing film according to claim
 1. 7. An optical laminate, comprising the thin polarizing film according to claim
 6. 